Optical storage device and liquid crystal device having separate liquid crystal layers with opposing electrodes

ABSTRACT

There are disclosed an optical storage device for accessing an optical recording medium such as a phase change type of optical disk and an optical magnetic disk, and a liquid crystal device preferably applicable to such a optical storage device. The liquid crystal device has a structure that two liquid crystal layers, each of which has stripe-like shaped electrodes arranged in a direction perpendicularly intersecting one another, are superposed. This structure makes it possible to effectively correct an aberration caused by a change in a depth from a surface of an optical recording medium to a condensing point, for example, in cases of unevenness in thickness of a protective layer and a multi-layer recording.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical storage device for accessingan optical recording medium such as a phase change type of optical diskand an optical magnetic disk, and a liquid crystal device preferablyapplicable to such a optical storage device.

2. Description of the Related Art

An optical disk, such as a phase change type (PD) of optical disk and anoptical magnetic disk (MO), is a portable recording medium having a highstorage capacity, and is being watched as a recording medium of apersonal computer. A possibility of higher density and larger capacityfor such an optical disk is pursued.

To implement a larger capacity of an optical disk while a recording areaof the optical disk is maintained as it is, there is a need to increasea recording density of the recording area or to establish a multi-layerconstruction of recording. In order to increase a recording density ofthe recording area, there is basically a need to reduce a condensingspot of a laser beam to be used, while there is considered means such asa magnetic resolution.

Generally, a diameter of a spot of a laser beam is in proportion to

λ/NA (λ:wavelength of light, NA: numerical aperture).

Therefore, to implement a higher density of recording for an opticaldisk, there is a need to use a laser (for example, a laser emittinglight of blue) which is short in wavelength, or increase NA of anobjective lens.

However, in the event that NA of an objective lens is increased, itinvolves a problem of a spherical aberration due to an unevenness inthickness of transparent protective layers on a surface of an opticaldisk when the optical disk is manufactured.

Particularly, since the optical disk is constructed as a storage mediumwhich is detachably loaded, there is needed a transparent protectivelayer on a layer which is essentially necessary for a storage and apick-up of information, such as a reflecting layer and a recordinglayer. The unevenness in thickness of the protective layer on themanufacture is of ±50 μm or so as an unevenness on an individual opticaldisk (an unevenness as an individual difference) and is of ±10 μm or soas a variation inside the same optical disk (an unevenness inside anindividual). The unevenness in thickness of transparent protectivelayers on a surface of an optical disk causes a spherical aberration onlight condensed on the recording layer, and this spherical aberrationhas a bad effect on recording and reading of a pit mark.

FIG. 1 is a diagram showing a spherical aberration RMS to an unevennessof a protective layer.

FIG. 1 shows results of calculations in case of NA=0.6 corresponding tothe present DVD, and in case of NA=0.85. In a case where an unevennessin thickness of the protective layer on the manufacture is of +S50 m, ifNA=0.6, it is within an aberration allowance. On the other hand, ifNA=0.85, an aberration, which cannot be covered, occurs. Therefore, toimplement a high NA of objective lens there is needed a mechanism forcorrecting the spherical aberration in accordance with an unevenness inthickness of the protective layer.

In order to satisfy such a requirement, there is proposed a scheme inwhich two objective lenses are used so that a spherical aberration isactively corrected by mechanically altering an interval between the twoobjective lenses (cf. Japanese Patent Laid Open Gazette Hei. 8-212579).

However, according to this proposal, a further mechanical driving forthe objective lens is added. Thus, this is associated with a problemthat a weight of a head portion of a pick-up is increased and a largerspace is needed.

On the other hand, there is proposed a scheme in which a liquid crystaldevice is disposed in an optical path so as to correct an aberration(cf. for example, Japanese Patent Laid Open Gazette Hei. 8-212611,Japanese Patent Laid Open Gazette Hei. 9-128785).

As such a liquid crystal device, there are known two types of anelectrode structure of two dimensional matrix configuration and anelectrode structure patterned after a pattern associated with theaberration.

To implement a matrix configuration of electrode, there is a need to usea TFT. The TFT matrix-panel needs a very complicated manufacturingprocess, and thus this is associated with such a problem that it isobliged to increase greatly the cost.

On the other hand, with respect to the electrode structure patterned, itis associated with a problem that a phase distribution of light, whichis formed by a distribution of index of refraction of the liquid crystaldevice, is fixed. Thus, there is a need to dispose the liquid crystaldevice as to an optical axis with great accuracy. This involves such aproblem that a strict precision of an alignment is required. In order tocorrect a spherical aberration through a patterned electrode structure,an electrode structure having a concentric circle of pattern is adopted.However, to correct the spherical aberration with greater accuracy, ifthe concentric circle of pattern is given with greater definition, thiscauses a polarization of the baser beam to be disturbed. Thus, this isnot suitable for correction of the spherical aberration when theobjective lens of a high NA is adopted. Further, in the event that theconcentric circle of pattern is given with greater definition, thiscauses the number of lead wires derived from a ring electrode inside theconcentric circle to be increased, and thereby increasing a wiring areafor the lead wires. This is associated with a problem on manufacturethat the concentric circle of pattern cannot be formed per se.

Further, Japanese Patent Laid Open Gazette Hei. 9-128785 discloses theuse of a strip shaped electrode. However, the use of a strip shapedelectrode cannot almost correct the aberration from a view point of anaberration correction at the time of a high NA.

In the above description, the necessity for the aberration correction isexplained in association with an unevenness in thickness of theprotective layer of the optical disk. On the other hand, also when it isintended that an optical storage medium having a multi-layerconstruction of recording, that is, a plurality of information recordinglayers in a depth direction, is implemented, there is a need to activelycorrect the aberration due to the variation in depth.

While the above explanation is made in connection with the optical disk,the above-mentioned problems are applied to, for example, a tape-likeshaped optical storage medium too, regardless of the disk configuration.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide an optical storage device capable of effectively correcting anaberration caused by a variation in a depth from a surface of an opticalstorage medium to a condensing point, even if the depth is varied, forexample, in cases of an unevenness in thickness of a protective layerand the multi-layer recording, and a liquid crystal device capable ofbeing preferably adopted to the optical storage device for a use of theaberration correction.

To achieve the above-mentioned objects, the present invention providesan optical storage device comprising:

a light source;

an irradiation optical system for leading light emitted from said lightsource to condense on a predetermined optical storage medium;

a photo detector for picking up a signal light carrying informationstored in said optical storage medium to read the information, saidsignal light being condensed onto said optical storage medium andreflected on said optical storage medium;

a pick-up optical system for leading said signal light to said photodetector;

a liquid crystal device having first and second liquid crystal layersdisposed in mid way of an optical path of said irradiation opticalsystem and extending in parallel with a direction intersecting saidoptical path, a plurality of first electrodes for driving said firstliquid crystal layer, said plurality of first electrodes extending apredetermined x-direction intersecting said optical path and arranged ina y-direction intersecting both said optical path and said x-direction,and a plurality of second electrodes for driving said second liquidcrystal layer, said plurality of second electrodes extending they-direction and arranged in the x-direction; and

a liquid crystal driver for applying controlled voltages to saidplurality of first electrodes and said plurality of second electrodes ofsaid liquid crystal device to correct an aberration of light to becondensed on said optical storage medium.

An important inventive feature of an optical storage device of thepresent invention resides in the point that the optical storage deviceadopts a liquid crystal device having a structure that two liquidcrystal layers, each of which has stripe-like shaped electrodes arrangedin a direction perpendicularly intersecting one another, are superposed.This feature makes drawing of lead wires from the electrodes easy andalso makes it possible to facilitate a fabrication of the device.Further, according to the optical storage device of the presentinvention, it is possible to correct the aberration in accordance withan electric control and thereby permitting a very rough alignment.Furthermore, according to the optical storage device of the presentinvention, as will be described later in connection with the preferredembodiments, the optical storage device has a sufficient aberrationcorrection ability.

In the optical storage device according to the present invention asmentioned above, it is acceptable that said irradiation optical systemhas an objective lens at a place adjacent to said optical storagemedium, said objective lens comprising a plano-convex lens and anaspherical lens.

This feature makes it possible to easily implement a high NA ofobjective lens, and thus according to the optical storage device of thepresent invention, it is possible to give a sufficient aberrationcorrection ability, even if a high NA of objective lens is used.

In the optical storage device according to the present invention asmentioned above, it is preferable that said liquid crystal driverapplies voltages to said plurality of first electrodes and saidplurality of second electrodes of said liquid crystal device, saidvoltages being controlled in such a manner that a phase distribution oflight passing through said first liquid crystal layer in the y-directionis of a Kinoform structure and a phase distribution of light passingthrough said second liquid crystal layer in the x-direction is of aKinoform structure.

This feature makes it possible to constitute the liquid crystal devicewith thin liquid crystal layers, and thereby contributing to higheroperational speed of the liquid crystal device.

Further, in the optical storage device according to the presentinvention as mentioned above, it is preferable that said first andsecond liquid crystal layers of said liquid crystal driver aredetermined in their properties in such a manner that a normal of analtering surface of a liquid crystal molecular alignment in said firstliquid crystal layer, due to a change in an electric field within saidfirst liquid crystal layer according to changes of voltages applied tosaid first electrodes, and a normal of an altering surface of a liquidcrystal molecular alignment in said second liquid crystal layer, due toa change in an electric field within said second liquid crystal layeraccording to changes of voltages applied to said second electrodes, aredirected to a same direction.

Such a determination of properties of the first and second liquidcrystal layers prevents a polarization state of the incident light frombeing changed by the liquid crystal itself.

Alternatively, in the optical storage device according to the presentinvention as mentioned above, it is preferable that said first andsecond liquid crystal layers of said liquid crystal driver aredetermined in their properties in such a manner that a normal of analtering surface of a liquid crystal molecular alignment in said firstliquid crystal layer, due to a change in an electric field within saidfirst liquid crystal layer according to changes of voltages applied tosaid first electrodes, and a normal of an altering surface of a liquidcrystal molecular alignment in said second liquid crystal layer, due toa change in an electric field within said second liquid crystal layeraccording to changes of voltages applied to said second electrodes,establish a predetermined angle (e.g. 90°), and a wavelength plate (e.g.λ/2 plate) for rotating a polarization direction of an incident light bythe predetermined angle is disposed between the first liquid crystallayer and the second liquid crystal layer.

This arrangement also prevents a polarization state of the incidentlight from being changed by the liquid crystal itself.

Further, in the optical storage device according to the presentinvention as mentioned above, it is preferable that properties inalignment of liquid crystal molecules of said first and second liquidcrystal layers of said liquid crystal device are of bend.

In a manufacturing process of a liquid crystal device, a property inalignment of liquid crystal molecules of liquid crystal layers of theliquid crystal device is selectable between a bend and a splay inaccordance with a direction for a rubbing (mechanically) a substrate forsupporting the liquid crystal layer. A selection of the bend contributesto a higher speed in change of alignment of liquid crystal molecules ofthe liquid crystal layer, that is, a higher operational speed of theliquid crystal device.

Furthermore, in the optical storage device according to the presentinvention as mentioned above, it is preferable that said irradiationoptical system is an optical system which permits light beams emittedfrom said light source to pass through said liquid crystal device by onetime while the light beams are condensed on said optical storage medium,and said first and second liquid crystal layers of said liquid crystaldevice are set up in their thickness such that phases of light emittedfrom said light source and passing through said first and second liquidcrystal layers vary between 0 and 2π under control of voltages appliedto said first electrodes and said second electrodes, respectively.

A provision of a thickness of a liquid crystal layer, which permits aphase of a light to vary between 0 and 2π, makes it possible to correctan aberration adopting a phase change of the above-mentioned Kinoformstructure, and also contributes to a higher speed in change of alignmentof liquid crystal molecules of the liquid crystal layer.

Still further, in the optical storage device according to the presentinvention as mentioned above, it is preferable that said irradiationoptical system is an optical system which permits light beams emittedfrom said light source to pass through said liquid crystal device on areciprocation basis while the light beams are condensed on said opticalstorage medium, and said first and second liquid crystal layers of saidliquid crystal device are set up in their thickness such that phases oflight emitted from said light source and passing through said first andsecond liquid crystal layers by one time vary between 0 and π undercontrol of voltages applied to said first electrodes and said secondelectrodes, respectively.

In this case, the thickness of the liquid crystal layers becomes furtherhalf and thereby implementing a higher speed operation.

Still further, in the optical storage device according to the presentinvention as mentioned above, it is preferable that said liquid crystaldevice has a reflecting surface for reflecting a light incident ontosaid liquid crystal device and passing through both said first andsecond liquid crystal layers and for causing the light to pass throughboth said first and second liquid crystal layers again.

In the event that the liquid crystal device is used on a reciprocationbasis, a provision of the above-mentioned reflecting surface on theliquid crystal device may avoid a necessity for preparing an additionalreflecting mirror or the like. Thus, it is possible to contribute tominiaturization and low cost.

Still further, in the optical storage device according to the presentinvention as mentioned above, it is preferable that a width of each ofsaid first electrodes of said liquid crystal device in connection withthe y-direction has a size not less than a thickness of said firstliquid crystal layer, and a width of each of said second electrodes ofsaid liquid crystal device in connection with the x-direction has a sizenot less than a thickness of said second liquid crystal layer.

In the event that a width of an electrode is narrower than a thicknessof a liquid crystal layer, an electric field, which is formed in theliquid crystal layer by a voltage applied to the electrode, extendswithin the liquid crystal layer. As a result, there is a possibilitythat an electric field distribution formed within the liquid crystallayer is greatly different from an ideal electric field distribution andthus it is difficult to expect a sufficient aberration correction. Onthe other hand, a formation of an electrode having a width larger thanthe thickness of the liquid crystal layer, as mentioned above, makes itpossible to form a suitable electric field distribution within theliquid crystal layer.

Still further, in the optical storage device according to the presentinvention as mentioned above, it is acceptable that a part of saidirradiation optical system is shared with a part of said pick-up opticalsystem, said liquid crystal device is disposed at a portion for commonuse of said irradiation optical system and said pick-up optical system,light beams emitted from said light source are condensed via said liquidcrystal device onto said optical storage medium, and the signal lightcarrying information stored in said optical storage medium, which iscondensed onto said optical storage medium and reflected on said opticalstorage medium, is led via said liquid crystal device to said photodetector. Or alternatively it is acceptable that a part of saidirradiation optical system is shared with a part of said pick-up opticalsystem, said liquid crystal device is disposed at a portion other than aportion for common use of said irradiation optical system and saidpick-up optical system, light beams emitted from said light source arecondensed via said liquid crystal device onto said optical storagemedium, and the signal light carrying information stored in said opticalstorage medium, which is condensed onto said optical storage medium andreflected on said optical storage medium, is led via an optical path,which is different from an optical path passing through said liquidcrystal device, to said photo detector.

What needs an aberration correction is mainly an irradiation opticalsystem side, either of the structures as mentioned above is acceptable.

Still further, in the optical storage device according to the presentinvention as mentioned above, it is preferable that said irradiationoptical system has a beam splitter for splitting a light or determininga travelling direction of a light, and said liquid crystal device andsaid beam splitter are formed in a unitary body.

In the even that the irradiation optical system has a beam splitter,when the liquid crystal device and the beam splitter are formed in aunitary body, it is possible to reduce the number of parts andcontribute to effective assembling and miniaturization of the device.

While the above explanation is concerned such a matter that the pick-upoptical system of the optical storage device leads light reflected onthe optical storage medium to the photo detector, it is acceptable toprovide such an arrangement that a transparent type of optical storagemedium is prepared, and the pick-up optical system of the opticalstorage device leads light passing through the optical storage medium tothe photo detector

In this case, there is provided an optical storage device comprising:

a light source;

an irradiation optical system for leading light emitted from said lightsource to condense on a predetermined optical storage medium;

a photo detector for picking up a signal light carrying informationstored in said optical storage medium to read the information, saidsignal light being condensed onto said optical storage medium andpassing through said optical storage medium;

a pick-up optical system for leading said signal light to said photodetector;

a liquid crystal device having first and second liquid crystal layersdisposed in mid way of an optical path of said irradiation opticalsystem and extending in parallel with a direction intersecting saidoptical path, a plurality of first electrodes for driving said firstliquid crystal layer, said plurality of first electrodes extending apredetermined x-direction intersecting said optical path and arranged ina y-direction intersecting both said optical path and said x-direction,and a plurality of second electrodes for driving said second liquidcrystal layer, said plurality of second electrodes extending they-direction and arranged in the x-direction; and

a liquid crystal driver for applying controlled voltages to saidplurality of first electrodes and said plurality of second electrodes ofsaid liquid crystal device to correct an aberration of light to becondensed on said optical storage medium.

In the optical storage device according to the present invention asmentioned above, it is acceptable that said optical storage medium has aplurality of information storage points in a depth direction, saidliquid crystal driver applies voltages, which are controlled inaccordance with condensing points in the depth direction of said opticalstorage medium, to the plurality of first electrodes and the pluralityof second electrodes of said liquid crystal device, respectively, sothat an aberration correction according to the condensing points in thedepth direction of said optical storage medium is performed, saidoptical storage device further comprising:

a second liquid crystal device having third and fourth liquid crystallayers disposed in mid way of an optical path of said pick-up opticalsystem and extending in parallel with a direction intersecting saidoptical path, a plurality of third electrodes for driving said thirdliquid crystal layer, said plurality of third electrodes extending apredetermined x′-direction intersecting said optical path and arrangedin a y′-direction intersecting both said optical path and saidx′-direction, and a plurality of fourth electrodes for driving saidforth liquid crystal layer, said plurality of fourth electrodesextending the y′-direction and arranged in the x′-direction; and

a second liquid crystal driver for applying voltages, which arecontrolled in accordance with the condensing points in the depthdirection of said optical storage medium, to said plurality of thirdelectrodes and said plurality of fourth electrodes of said second liquidcrystal device to perform an aberration correction according to thecondensing points in the depth direction of said optical storage medium.

The present invention is also applicable to a multi-layer recordingscheme of optical storage medium, that is, an optical storage mediumhaving a plurality of information storage points in a depth direction.

Further, an optical storage device according to the present invention,it is possible to arrange it as an information writing dedicated-deviceto an optical storage medium.

In this case, there is provided an optical storage device comprising:

a light source;

an irradiation optical system for leading light emitted from said lightsource to condense on a predetermined optical storage medium;

a liquid crystal device having first and second liquid crystal layersdisposed in mid way of an optical path of said irradiation opticalsystem and extending in parallel with a direction intersecting saidoptical path, a plurality of first electrodes for driving said firstliquid crystal layer, said plurality of first electrodes extending apredetermined x-direction intersecting said optical path and arranged ina y-direction intersecting both said optical path and said x-direction,and a plurality of second electrodes for driving said second liquidcrystal layer, said plurality of second electrodes extending they-direction and arranged in the x-direction; and

a liquid crystal driver for applying controlled voltages to saidplurality of first electrodes and said plurality of second electrodes ofsaid liquid crystal device to correct an aberration of light to becondensed on said optical storage medium.

In the optical storage device according to the present invention asmentioned above, it is acceptable that said optical storage medium has aplurality of information storage points in a depth direction, and

said liquid crystal driver applies voltages, which are controlled inaccordance with condensing points in the depth direction of said opticalstorage medium, to the plurality of first electrodes and the pluralityof second electrodes of said liquid crystal device, respectively, sothat an aberration correction according to the condensing points in thedepth direction of said optical storage medium is performed.

To achieve the above-mentioned objects, the present invention provides aliquid crystal device comprising:

first and second liquid crystal layers extending in a state that theyare opposite to one another in parallel with a predetermined planeextending in an x-direction and a y-direction which intersect eachother;

a plurality of first electrodes for driving said first liquid crystallayer, said plurality of first electrodes extending the x-direction andarranged in the y-direction; and

a plurality of second electrodes for driving said second liquid crystallayer, said plurality of second electrodes extending the y-direction andarranged in the x-direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a spherical aberration RMS to an unevennessof a protective layer.

FIG. 2 is a graph showing a residual wave aberration (the axis ofordinate) to a variation (the axis of abscissas) in thickness of aprotective layer of an optical disk.

FIG. 3 is a schematic construction view of an optical storage deviceaccording to the first embodiment of the present invention.

FIG. 4 is a sectional view showing portions of a liquid crystal deviceand an objective lens in the optical storage device shown in FIG. 3.

FIG. 5 is a perspective view showing portions of a liquid crystal deviceand an objective lens in the optical storage device shown in FIG. 3.

FIGS. 6(A) and 6(B) are views showing electrode structures of a liquidcrystal device.

FIG. 7 is a graph showing an effect of correction of a sphericalaberration by a liquid crystal device.

FIG. 8 is a typical illustration showing an initial alignment of liquidcrystal molecules.

FIG. 9 is a typical illustration showing alignment characteristics ofliquid crystal molecules by an electric field.

FIG. 10 is an illustration showing a relationship between an electrodestructure of a liquid crystal device and an alignment surface of liquidcrystal molecules in a liquid crystal layer.

FIG. 11 is an illustration showing an alternative relationship betweenan electrode structure of a liquid crystal device and an alignmentsurface of liquid crystal molecules in a liquid crystal layer.

FIG. 12 is a graph showing a maximum phase control amount necessary forcorrection of a spherical aberration in a liquid crystal device.

FIG. 13 is an explanatory view useful for understanding a Kinoformstructure.

FIGS. 14(A) and 14(B) are views showing a relation between a thickness tof a liquid crystal layer and a width d of a stripe-like shapedelectrode.

FIG. 15 is a schematic construction view of an optical storage deviceaccording to the second embodiment of the present invention.

FIG. 16 is a schematic construction view of an optical storage deviceaccording to the third embodiment of the present invention.

FIG. 17 is a partially typical illustration of the second liquid crystallayer side of the liquid crystal device shown in FIG. 16.

FIG. 18 is a schematic construction view of an optical storage deviceaccording to the fourth embodiment of the present invention.

FIG. 19 is a schematic construction view of an optical storage deviceaccording to the fifth embodiment of the present invention.

FIG. 20 is a schematic construction view of an optical storage deviceaccording to the sixth embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, first, there will be described a theory as to a matter thataccording to a liquid crystal device, it is possible to effectivelycorrect an aberration caused by a variation in depth from a surface ofan optical storage medium to a condensing point inside the opticalstorage medium, and then there will be described embodiments of thepresent invention.

Incidentally, according to the following theoretical explanation, it isassumed that the condensing point is varied in depth owing to avariation in thickness of a protective layer of a surface of an opticaldisk.

A defocus aberration, which is owing to a shift of a focal point in anoptical axis direction, is caused by a simple variation of a referencespherical surface. Thus, it is possible to distinguish the defocusaberration from other aberration. In the conventional optical system ofa low NA, it is acceptable that the defocus aberration is in proportionto (NA)². However, in case of a high NA (here NA=0.85 is considered),there is a need to consider a high degree not less than 8 degree.Generally, a disturbance near the focal point is expressed by thefollowing equation. $\begin{matrix}{{u\left( {x,y} \right)} = {\int{\int_{{\xi^{2} + \eta^{2}} \leq {NA}^{2}}{{\exp \left\lbrack {{- }\quad {k\left( {{\xi \quad x} + {\eta \quad y} + {\sqrt{1 - \xi^{2} - \eta^{2}}z}} \right)}} \right\rbrack}{\xi}{\eta}}}}} & (1)\end{matrix}$

where (x, y) denotes coordinates, (ε, η) denotes coordinates on anaperture in a front side focal plane, and k denotes wave number.

At that time,

exp[−ik({square root over (1−ε²−η²)}z)]

is regarded as a wave distortion in a case where an image surface isshifted from a focal plane by a distance z. When ε²+η²=NA² is expressedand the shift of the distance is replaced by dz, the defocus aberrationdWdefocus is expressed by the following expression. $\begin{matrix}\begin{matrix}{{{Wdefocus}} = \quad {{z\left( {\sqrt{1 - {NA}^{2}} - 1} \right)}}} \\{= \quad {{\left( \frac{1}{2} \right){{{z({NA})}^{2}} \cdot r^{2}}} + {\left( \frac{1}{8} \right){{{z({NA})}^{4}} \cdot r^{4}}} +}} \\{\quad {{\left( \frac{1}{16} \right){{{z({NA})}^{6}} \cdot r^{6}}} + {\left( \frac{5}{128} \right){{{z({NA})}^{8}} \cdot r^{8}}} +}} \\{\quad {\left( \frac{7}{256} \right){{{z({NA})}^{10}} \cdot r^{10}}}}\end{matrix} & (1)\end{matrix}$

On the other hand, a spherical aberration due to a variation inthickness of a protective layer of an optical disk is obtained in thefollowing manner.

A movement Lz of a luminous flux of an emitting angle from a paraxialfocal point position when it passes through a disk substrate, due todisk thickness error Δt is expressed by${{Lz} = {\frac{d}{n}\left( {1 - \frac{n\quad \cos \quad \theta}{\sqrt{n^{2} - {\sin^{2}\theta}}}} \right)}},$

where, n is a refractive index of the substrate. In the event that sin θis developed up to 8 degree of item, and a lateral aberration of a rayof light is determined and is converted to a wave aberration, aspherical aberration dWdisk due to a variation in thickness of a disksubstrate, taking a high degree item, is expressed by an equation (2)$\begin{matrix}\begin{matrix}{{{Wdisk}} = \quad {{\frac{1}{4}\left( {\frac{- 1}{2n^{3}} + \frac{1}{2n}} \right){{t} \cdot {NA}^{4} \cdot r^{4}}} +}} \\{\quad {{\frac{1}{6}\left( {\frac{- 3}{8n^{5}} + \frac{1}{4n^{3}} + \frac{1}{8n}} \right){{t} \cdot {NA}^{6} \cdot r^{6}}} +}} \\{\quad {{\frac{1}{8}\left( {\frac{- 5}{16n^{7}} + \frac{3}{16n^{5}} + \frac{1}{16n^{3}} + \frac{1}{16n}} \right){{t} \cdot ({NA})^{8} \cdot r^{8}}} +}} \\{\quad {\frac{1}{10}\left( {\frac{- 35}{128n^{9}} + \frac{5}{32n^{7}} + \frac{3}{64n^{5}} + \frac{1}{32n^{3}} + \frac{5}{128n}} \right){{t} \cdot ({NA})^{10} \cdot r^{10}}}}\end{matrix} & (2)\end{matrix}$

where dt denotes a variation in thickness of a protective layer of thedisk substrate.

When a phase distribution, which is intended to be produced by a liquidcrystal, is represented by a phase transfer function, it is given byeven functions on x and y, and thus generally, they are expressed byfollowing equations (3) and (4).

 φ(x)=c ₁ ·x ² +c ₂ ·x ⁴ +c ₃ ·x ⁶ +c ₄ ·x ⁸ +c ₅ ·x ¹⁰  (3)

φ(y)=c ₁ ·y ² +c ₂ ·y ⁴ +c ₃ ·y ⁶ +c ₄ ·y ⁸ +c ₅ ·y ¹⁰  (4)

To correct a spherical aberration caused by a variation in thickness ofa protective layer of an optical disk with respect to an x-direction anda y-direction independently, the aberration is cancelled in accordancewith equations (3) and (4), while generated aberrations on equation (2)are regulated in accordance with equation (1). However, it is difficultto completely suppress the aberration to be zero, and the residualaberration remains. If the residual aberration is expressed by dW, thefollowing expression consists. W = Wdisk + Wdefocus − φ(x) − φ(y)$\left( {r = \sqrt{\left( {x^{2} + y^{2}} \right)}} \right)$

it is satisfactory that the residual aberration dW is sufficiently smallas compare with dWrms of an RMS value of a liquid aberration. A relationbetween the dWrms of the RMS value and the aberration dW is given asfollow.$\left( {{Wrms}} \right)^{2} = {\frac{\int_{0}^{1}{\int_{0}^{2\pi}{\left( {{W} - \overset{\_}{W}} \right)^{2}\rho {\rho}{\theta}}}}{\int_{0}^{1}{\int_{0}^{2\pi}{\rho {\rho}{\theta}}}} = {\overset{\_}{W^{2}} - \left( \overset{\_}{W} \right)^{2}}}$

ρ, θ are parameters wherein positions in the associated apertures arerepresented by polar coordinates, respectively. A relation between avariation dt in thickness of a protective layer of an optical disk indWdisk and a variation dz in free space distance in dWdefocus isexpressed by dz=K·dt where k is a coefficient. The residual aberrationcan be determined by means of applying a least square fitting, whereinparameters K, c₁˜c₅ are established, and an evaluation function is givenin the form of an RMS value of an aberration.

FIG. 2 is a graph showing a residual wave aberration (the axis ofordinate) to a variation (the axis of abscissas) in thickness of aprotective layer of an optical disk, determined in accordance with amanner as mentioned above. The used wavelength λ is 685 nm.

From this result, it would be understood that the use of a scheme ofindependent correction with respect to the x-direction and they-direction makes it possible in principle to keep the sphericalaberration within an allowance limit, even if a thickness of aprotective layer of an optical disk is varied ±600 μm.

FIG. 3 is a schematic construction view of an optical storage deviceaccording to the first embodiment of the present invention.

A laser beam emitted from a semiconductor laser 11 passes through acondenser lens 12 and a polarization beam splitter 13, and reflects on areflecting mirror 14, and further passes through a liquid crystal device10 and an objective lens 15, and goes toward an optical disk 100. Theoptical disk 100 has a transparent protective layer 100 a on a surfacethereof. Light beams emitted from the objective lens 15 are condensed ona point on a recording layer 100 b placed below the protective layer 100a. The unevenness in thickness of the protective layer 100 a on themanufacture is of the maximum ±50 μm or so as an unevenness on theindividual optical disk 100. A spherical aberration of light beamscondensed on the recording layer 100 b, which are owing to theunevenness in thickness of the protective layer 100 a, are corrected bythe liquid crystal device 10.

A signal light reflected on the recording-layer 100 b of the opticaldisk 100, which carries information recorded on the optical disk 100,passes through the objective lens 15 and the liquid crystal device 10,reflects on the reflecting mirror 14, enters the polarization beamsplitter 13, and goes to a beam splitter 16 side. An incident light tothe beam splitter 16 is divided into two parts one of which entersWollaston prism 17 whereby the light is separated in accordance with thepolarization direction. And the light thus separated enters via a lens18 a photo detector 19 for picking up information recorded on theoptical disk 100.

On the other hand, another of the two parts of light divided by the beamsplitter 16 enters via a lens 20 a beam splitter 21 wherein the light isfurther divided into two parts one of which enters a photo detector 22for a tracking error detection, and another enters a wedge prism 23wherein a light beam is further divided into two parts and is projectedonto a photo detector 24 for a focus error detection. The tracking errordetection, the focus error detection, and the optical system but theliquid crystal device 10 are of a well known technique per se, and thusa redundant description will be omitted.

FIG. 4 is a sectional view showing portions of the liquid crystal deviceand the objective lens in the optical storage device shown in FIG. 3.FIG. 5 is a perspective view showing portions of the liquid crystaldevice and the objective lens in the optical storage device shown inFIG. 3. FIGS. 6(A) and 6(B) are views showing electrode is structures ofthe liquid crystal device.

The objective lens 15 comprises a plano-convex lens 151 disposed at aposition coming very close to the protective layer 100 a, and anaspherical lens 152 disposed behind the plano-convex lens 151. It ispossible to constitute the objective lens 15 with a piece of lens.However, it is difficult to implement a high NA. For this reason,according to the present embodiment, a high NA of objective lens isconstructed through a combination of the plano-convex lens 151 and theaspherical lens 152 as shown in FIG. 4. The objective lens 15 iscontrolled in movement in such a manner that a movement as to adirection parallel to a surface of the optical disk, for example, adirection perpendicular to a sheet face of FIG. 4, is controlled inaccordance with the tracking error signal derived from the photodetector 22 for a tracking error detection as shown in FIG. 3, and amovement as to a direction approaching or 5 separating from the opticaldisk, that is, a vertical direction of FIG. 4, is controlled inaccordance with the focus error signal derived from the photo detector24 for a focus error detection.

The liquid crystal device 10 is disposed behind the objective lens 15.

The liquid crystal device 10 has, as shown in FIG. 4, a structure that afirst liquid crystal layer 102 is sandwiched between a glass substrate101 and a glass substrate 103, and a second liquid crystal layer 104 issandwiched between a glass substrate 103 and a glass substrate 105. Asshown in FIG. 6(A), the first liquid crystal layer 102 is sandwichedbetween a plurality of transparent first electrodes 106 each extendingin an X-direction and arranged in a Y-direction and a transparent solidelectrode (not illustrated) extending to the whole surface. As shown inFIG. 6(B), the second liquid crystal layer 104 is sandwiched between aplurality of transparent second electrodes 107 each extending in aY-direction and arranged in an X-direction and a transparent solidelectrode (not illustrated) extending to the whole surface. Tomanufacture the liquid crystal device 10, the respective electrode isformed on a surface of the associated glass substrate, and then theassociated alignment film for determining an alignment for a liquidcrystal is formed on the electrode thus formed. The respective liquidcrystal layer is formed in a state that it is sandwiched between thealignment films. As shown in FIGS. 6(A) and 6(B), the first electrodes106 for driving the first liquid crystal layer and the second electrodes107 for driving the second liquid crystal layer are extended in adirection perpendicularly intersecting to one another and are arrangedin a direction perpendicularly intersecting to one another.Incidentally, the circles depicted by a broken line, as shown in FIGS.6(A) and 6(B), denote the passages of light beams.

The plurality of first electrodes 106 shown in FIG. 6(A) are connectedvia the associated pads 108 and the associated lead wires 109 shown inFIG. 5 to a drive circuit 50. Likely, the plurality of second electrodes107 shown in FIG. 6(B) are connected via the associated pads 110 and theassociated lead wires 111 shown in FIG. 5 to the drive circuit 50. Thedrive circuit 50 applies controlled voltages between the plurality offirst electrodes 106 of the liquid crystal device 10 and the solidelectrode, and between the plurality of second electrodes 107 of theliquid crystal device 10 and the solid electrode, so that an aberrationof light beams condensed on the optical disk is corrected. According tothe present embodiment (FIG. 3), when the optical disk 100 is loadedonto the optical storage device according to the present embodiment, acontrol voltage necessary for a suitable aberration correction isapplied to the liquid crystal device 10 to monitor a focus error signalderived from the photo detector 24. At that time, a voltage to beapplied to the liquid crystal device 10 is varied so that an applyingvoltage to the liquid crystal device 10, with which an appropriate focuserror signal is obtained, is given in the form of a necessary aberrationcorrection amount. Thus, it is possible to know a varying amount of theprotective layer 100 a on a surface of the optical disk 100.

When the optical disk 100 is actually accessed, the drive circuit 50drives the liquid crystal device 10 to correct a spherical aberration inaccordance with a detected thickness. Incidentally, according to thepresent embodiment, an unevenness (an unevenness inside an individual)in thickness of the protective layer inside the single optical disk isneglected, since it is very small as compared with an unevenness as anindividual difference.

FIG. 7 is a graph showing an effect of correction of a sphericalaberration by the liquid crystal device 10 wherein as the objective lens15 shown in FIG. 4, an objective lens adopting parameters of Table 1 setforth below is constructed.

FIG. 7 shows a residual spherical aberration wherein a protective layeron a surface of an optical disk is varied actually in thickness,assuming that the reference thickness of the protective layer is 0.6 mm,and the objective lens is designed, manufactured and disposed so as tobring about no spherical aberration when the reference thickness of theprotective layer is 0.6 mm.

TABLE 1 Wavelength 685 nm NA (numerical aperture) 0.85 Aspherical lensCurvature radius 3.0 mm (aspheric surface) Thickness 2.6 mm Refractiveindex 1.511 Plano-convex lens Curvature radius 1.25 mm Thickness 1.4 mmRefractive index 1.513 Focal length 4.0 mm

Here, as shown in FIGS. 6(A) and 6(B), the first electrodes 106 and thesecond electrodes 107 are arranged as a stripe. Distributions of thefirst and second liquid crystal layers 102 and 104 offer a step-likeshaped one on each arrangement pitch of the first electrodes 106 and thesecond electrodes 107. However, according to this simulation, the phasedistributions of light by the first and second liquid crystal layers 102and 104 constituting the liquid crystal device are given in form ofcontinuous amount, but not such a step-like shaped one.

Further, here, the first and second liquid crystal layers 102 and 104are different in a position of an optical axis direction. Thus, insteadof the above-mentioned equations (3) and (4), the following equations(5) and (6) are adopted.

φ(x)=c ₁ ·x ² +c ₂ ·x ⁴ +c ₃ ·x ⁶ +c ₄ ·x ⁸ +c ₅ ·x ¹⁰  (5)

φ(y)=c ₁ ·y ² +c ₂ ·y ⁴ +c ₃ ·y ⁶ +c ₄ ·y ⁸ +c ₅ ·y ¹⁰  (6)

According to the present embodiment, the first electrodes 106 and thesecond electrodes 107, which are arranged as a stripe as shown in FIGS.6(A) and 6(B), are adopted, and thus light beams passing through theliquid crystal device are subjected to a phase distribution correctionindependent on a primary dimensional basis with respect to anX-direction and a Y-direction. Nevertheless in the event that the phasedistribution correction as to the X-direction and the phase distributioncorrection as to the Y-direction are synthesized, a phase, which isreversed from the spherical aberration, is formed, so that the sphericalaberration is corrected.

The residual aberration distribution after correction offers a resultthat an RMS value is reduced under a balance of the vertical andhorizontal directions (X-direction and Y-direction) and the obliquedirection. The RMS value is sufficiently small, as shown in FIG. 7, ascompared with an allowance (0.07λ), even if the thickness of theprotective layer varies ±50 μm (±0.05 mm) or so. Thus, this involves noproblem with respect to an imaging performance of a condensing spot.

FIG. 8 the is a typical illustration showing an initial alignment ofliquid crystal molecules.

As the initial alignment of liquid crystal molecules, there are ahorizontal alignment (part (A)), HAN (part (B)) and a vertical alignment(part (C)). It is acceptable that any one of those alignments isadopted. It is noted that since it is necessary that a polarizationstate of a laser beam is not varied by a liquid crystal per se, there isa need that a varying face of an alignment of liquid crystal moleculesby an electric field is identical to a polarization direction of a laserbeam passing through the liquid crystal layer.

FIG. 9 is a typical illustration showing alignment characteristics ofliquid crystal molecules by an electric field.

The alignment characteristic of liquid crystal molecules by an electricfield is classified, as shown in FIG. 9, into a splay type and a bendtype. In the event that an alignment processing involving a mechanicalrubbing, which is referred to as “rubbing” in a manufacturing process ofa liquid crystal device, is applied, the splay type is defined byrubbing upper and lower substrates sandwiching a liquid crystal layer inmutually different directions, and the bend type is defined by rubbingin the same direction. In the event that a change of liquid crystalmolecules in alignment Makes the same phase alteration on light beamspassing through the liquid crystal layer, the bend alignment offershigher response as compared with the splay alignment. Therefore,according to the present embodiment, the bend alignment is adopted forboth the first liquid crystal layer and the second liquid crystal layer,and thereby contributing to higher operating speed.

FIG. 10 is an illustration showing a relation between an electrodestructure of a liquid crystal device and an alignment surface of liquidcrystal molecules in a liquid crystal layer.

Now it is assumed that a longitudinal direction (Y-direction) of thesecond electrodes 107 for driving one (here the second liquid crystallayer 104) of the first and second liquid crystal layers 102 and 104constituting the liquid crystal device 10 is identical to thepolarization direction of the laser beams passing through the liquidcrystal layer, and a longitudinal direction (X-direction) of the firstelectrodes 106 for driving another (here the first liquid crystal layer102) perpendicularly intersects to the polarization direction of thelaser beams passing through the liquid crystal layer. Further, it isassumed that an alignment surface of liquid crystal molecules is thesame direction (Y-direction) between the first and second liquid crystallayers 102 and 104, and is the same direction as the polarizationsurface of the laser beams. This arrangement makes it possible toprevent the polarization direction of the laser beams from beingdisturbed by the liquid crystal device.

FIG. 11 is an illustration showing an alternative relation between anelectrode structure of a liquid crystal device and an alignment surfaceof liquid crystal molecules in a liquid crystal layer.

According to the embodiment shown in FIG. 11, an alignment surface ofliquid crystal molecules in the first liquid crystal layer 102 is thealignment direction (Y-direction) of the first electrodes 106, and analignment surface of liquid crystal molecules in the second liquidcrystal layer 104 is the alignment direction (X-direction) of the secondelectrodes 107. This means, if it is maintained, that the polarizationsurface of the laser beams and the alignment surface of liquid crystalmolecules would intersect on either the first liquid crystal layer 102or the second liquid crystal layer 104, but not coincide, even if thepolarization surface of the laser beams is directed to either theX-direction or the Y-direction. For this reason, according to thepresent embodiment, there is disposed a λ/2 plate 112 between the firstliquid crystal layer 102 and the second liquid crystal layer 104. Theλ/2 plate 112 serves to rotate the alignment surface of the laser beamspassing therethrough by 90° . Thus, if the polarization surface of lightbeing applied from under FIG. 11 to the liquid crystal device 10 is setto be coincident with the alignment surface of liquid crystal moleculesof the second liquid crystal layer 104, the laser beams passed throughthe second liquid crystal layer 104 rotates by 90° at the λ/2 plate 112,so that the polarization surface of the laser beams is coincident withthe alignment surface of liquid crystal molecules of the first liquidcrystal layer 102, and whereby the laser beams pass through the firstliquid crystal layer 102. Consequently, also in the embodiment shown inFIG. 11, the polarization surface of the laser beams and the alignmentsurface of liquid crystal molecules in both the first liquid crystallayer 102 and the second liquid crystal layer 104 coincide, and thus itis possible to prevent the polarization of the laser beams from beingdisturbed by the liquid crystal device 10.

FIG. 12 is a graph showing a maximum phase control amount necessary forcorrection of a spherical aberration in a liquid crystal device, whereina reference thickness of a protective layer of an optical disk substrateis 0.6 mm, a thickness of the protective layer is varied between 0.4 mmand 0.8 mm.

As shown in FIG. 12, the retadation, where a reference thickness of aprotective layer of an optical disk substrate is varied ±0.2 mm (0.4mm˜0.8 mm), is about ±40λ. Even in case of ±50 μm (±0.05 mm) which isconsidered as an unevenness in thickness of the protective layer on eachindividual of the optical disk, it is about ±10λ. When it is intended tofaithfully control ±10λ, there is a need to prepare a thick liquidcrystal layer. The use of a thick liquid crystal layer brings aboutdecreasing the response speed and also increasing a cost. For thisreason, according to the present embodiment, a phase distribution withina beam plane of laser beams is controlled to be a phase distribution ofa Kinoform structure which will be described hereinafter.

FIG. 13 is an explanatory view useful for understanding a Kinoformstructure.

The Kinoform structure is a phase structure using a principle of afresnel lens wherein a phase is replaced by a phase 0 every phase 2π.Adoption of the Kinoform structure makes it possible to vary a phase oflaser beams between 0 and 2π by means of forming a liquid crystal layerof 8 μm or so in thickness in the event that a liquid crystal materialof Δn=0.15 is used where Δn denotes a difference in refractive indexbetween a case where a voltage is applied to electrodes and a case whereno voltage is applied to electrodes. In the event that a phase gradationbetween 0 and 2π is set up to 8 levels, a pitch of stripe-like shapedelectrodes is given with fineness of 12 μm to 13 μm or so.

A voltage to be applied to the electrodes thus formed is determined by aspherical aberration (or a thickness of a protective layer on an opticaldisk surface) to be corrected. Specifically, applying voltages to therespective electrodes are determined in accordance with theabove-mentioned equations (5) and (6) wherein coefficients c₁˜c₁₀, whichare determined in accordance with the spherical aberration (or athickness of a protective layer on an optical disk surface) to becorrected, are substituted for the equations (5) and (6), and inaddition a varying characteristic of a refractive index of a liquidcrystal layer to an applying voltage, which is determined by propertiesof a liquid crystal used in the liquid crystal layer.

FIGS. 14(A) and 14(B) are views showing a relation between a thickness tof a liquid crystal layer and a width d of a stripe-like shapedelectrode.

FIG. 14(A) shows a case where a width d of the electrode to which avoltage for driving the liquid crystal layer is applied is larger than athickness t of the liquid crystal layer. In this case, an electricfield, which is formed within the liquid crystal layer when a voltage isapplied to the electrode, is formed substantially along the electrode,as shown with the broken lines in FIG. 14(A).

FIG. 14(B) shows a case where a width d of the electrode to which avoltage for driving the liquid crystal layer is applied is smaller thana thickness t of the liquid crystal layer.

In this case, an electric field, which is formed within the liquidcrystal layer when a voltage is applied to the electrode, is expandedmore than a width d of the electrode to which the voltage is applied,and thus there is a possibility that an expected electric field cannotbe formed within the liquid crystal layer.

Thus, there is a limit in narrowing the pitch of the electrodes tostrictly meet the equations (5) and (6), and therefore it is preferablethat the liquid crystal device is formed in such a manner that the awidth d of the electrode is larger than a thickness t of the liquidcrystal layer.

FIG. 15 is a schematic construction view of an optical storage deviceaccording to the second embodiment of the present invention.

A laser beam emitted from a semiconductor laser-servo detection systemunited body type of device 11′ passes through the condenser lens 12 andthe liquid crystal device 10, and enters a beam splitter 25. The laserbeam arrived at the beam splitter 25 passes through the beam splitter25, and reflects on the mirror 14, and further passes through theobjective lens 15, and goes toward the optical disk 100. Light beamsemitted from the objective lens 15 are condensed on a point on therecording layer 100 b placed below the protective layer 100 a of theoptical disk 100. A signal light reflected on the recording layer 100 bof the optical disk 100, which carries information recorded on theoptical disk 100, passes through the objective lens 15, reflects on themirror 14, enters the beam splitter 25. An incident light to the beamsplitter 25 is divided into two parts one of which is a signal lightgoing to the united body type of device 11′, and another a signal lightgoing to the photo detector 24 for reading information recorded on theoptical disk 100. The signal light directed to the united body type ofdevice 11′ passes through the liquid crystal device 10 and the condenserlens 12, and enters the united body type of device 11′. The united bodytype of device 11′ generates a tracking error signal and a focus errorsignal in accordance with the incident light, so that the objective lens15 is driven in accordance with those error signals.

On the other hand, the signal light outgoing to the photo detector 24side enters the photo detector 24 via Wollaston prism 26. The photodetector 24 reads information recorded on the optical disk 100.

According to the present embodiment, the liquid crystal device 10 isdisposed at a portion (specifically, between the condenser lens 12 andthe beam splitter 25) other than a portion for a common use with apick-up optical system for introducing the signal light reflected on theoptical disk 100 to the photo detector 24 for information reading, of anillumination optical system up to a process of outgoing of laser beamsemitted from the united body type of device 11′ from the objective lens15. Light beams emitted from the united body type of device 11′ arecondensed via the liquid crystal device 10 on the optical disk 100. Thesignal light carrying information stored in the optical disk 100 is leadto the photo detector 24 without passing through the liquid crystaldevice 10.

In the event that an optical magnetic disk is adopted as the opticaldisk 100, it is necessary for the optical storage device to detect arotatory polarization of Kerr effect due to magnetism. Thus, in theevent that the liquid crystal device has an affect on a detection ofKerr effect, it is preferable that the liquid crystal device is disposedat a position that the signal light reflected on the optical disk 100does not pass through the liquid crystal device again, as shown in FIG.15. On the other hand, in the event that the liquid crystal device hasno affect on the signal light reflected on the optical disk 100, it isacceptable that the liquid crystal device is disposed at the position asshown in FIG. 3. In this case, it is preferable that the liquid crystaldevice 10 and the objective lens 15 are formed in a unitary body.

FIG. 16 is a schematic construction view of an optical storage deviceaccording to the third embodiment of the present invention.

A laser beam emitted from the semiconductor laser-servo detection systemunited body type of device 11′ passes through the condenser lens 12, andenters a beam splitter 27, and then goes to a liquid crystal device 10′.

FIG. 17 is a partially typical illustration of the second liquid crystallayer side of the liquid crystal device 10′ shown in FIG. 16.

A laser beam emitted from the beam splitter 27 shown in FIG. 16 to theliquid crystal device 10′ side enters the liquid crystal device 10′ in adirection directed from above to below, passes through a first liquidcrystal layer (cf. FIG. 10) not illustrated in FIG. 17, and then passesthrough the second liquid crystal layer 104 illustrated in FIG. 17. Atthe bottom of the second liquid crystal layer 104, there is formed analuminized film as an electrode. The aluminized film serves as not onlya solid electrode 113 but also a reflecting mirror. A laser beam passedthrough the second liquid crystal layer 104 and reached the solidelectrode 113 reflects on the solid electrode 113, again passes throughthe second liquid crystal layer 104, again passes through the firstliquid crystal layer not illustrated, and goes out from a surface of theliquid crystal device 10′, which surface is the same as an incidentsurface of the liquid crystal device 10′ which the laser beam is appliedto. In this manner, according to the liquid crystal device 10′ of thepresent embodiment, the laser beam passes through the first liquidcrystal layer and the second liquid crystal layer twice, respectively.Consequently, it is sufficient for the first liquid crystal layer andthe second liquid crystal layer to have a thickness which permits aphase of the laser beam for a once passage to vary between 0 and π.Thus, according to the present embodiment, as compared with theembodiments shown in FIGS. 3 and 15, it is possible to reduce athickness of the liquid crystal layer to a further half and therebycontributing to a higher speed operating.

In the third embodiment shown in FIG. 16, a laser beam emitted from theliquid crystal device 10′ passes through the beam splitter 27 andanother beam splitter 25, and reflects on the mirror 14, and furtherpasses through the objective lens 15, and goes toward the optical disk100. Light beams emitted from the objective lens 15 are condensed on therecording layer 100 b placed below the protective layer 100 a. Thesignal light reflected on the recording layer 100 b passes through theobjective lens 15, reflects on the mirror 14, and enters the beamsplitter 25. The signal light entered the beam splitter 25 is split intotwo parts of a signal light directed toward the united body type ofdevice 11′ and a signal light directed toward the photo detector 24 forreading information recorded on the optical disk 100. The signal lightdirected to the united body type of device 11′ passes through the beamsplitter 27 to reciprocate the liquid crystal device 10′ by one turn,reflects on the beam splitter 27, and enters the united body type ofdevice 11′. The united body type of device 11′ generates a trackingerror signal and a focus error signal in accordance with the incidentlight, so that the objective lens 15 is driven in accordance with thoseerror signals.

On the other hand, the signal light outgoing to the photo detector 24side enters the photo detector 24 via Wollaston prism 26. The photodetector 24 reads information recorded on the optical disk 100.

FIG. 18 is a schematic construction view of an optical storage deviceaccording to the fourth embodiment of the present invention. Onlydifferent points from the third embodiment shown in FIG. 16 will bedescribed.

According to the embodiment shown in FIG. 18, the beam splitter 27 andthe liquid crystal device 10′ are formed in a unitary body. Thisarrangement makes it possible to reduce the number of parts andcontribute to miniaturization. Other contractual portions of theembodiment shown in FIG. 18 are the same as the third embodiment shownin FIG. 16, and thus a redundant description will be omitted.

Forming the beam splitter and the liquid crystal device in a unitarybody is applicable also to the embodiment shown in FIG. 15. In case ofthe embodiment shown in FIG. 15, the beam splitter 25 and the liquidcrystal device 10 are formed in a unitary body.

FIG. 19 is a schematic construction view of an optical storage deviceaccording to the fifth embodiment of the present invention.

This embodiment relates to a scheme of picking up transmitted light ofan optical storage medium 100′.

A laser beam emitted from the semiconductor laser 11 passes through thecondenser lens 12, a phase plate 28, the liquid crystal device 10 and anobjective lens 15, and enters the optical storage medium 100′. Theoptical storage medium 100′ has a plurality of information storagepoints in a depth direction (a vertical direction of FIG. 19). In thiscase, the spherical aberration is varied in accordance with a depthposition of an information storage point for information to be writtenor read out. For this reason, according to the present embodiment, adriving circuit 50 for driving the liquid crystal device 10 drives theliquid crystal device 10, taking into consideration a position of anaccess point in a depth direction, in such a manner that the sphericalaberration (a convergence side spherical aberration) becomes alwaysminimum.

Light beams transmitted through the optical storage medium 100′ passthrough a condenser lens 31 and an additional liquid crystal device 32,reflect on a reflecting mirror 33, further pass through a lens 34 and azone plate 35, and enter a pin-hole type of photo detector 36, so thatinformation stored in the optical storage medium 100′ is read inaccordance with the same principle as a phase-contrast microscope.

The liquid crystal device 32, which is provided in a pick-up opticalsystem side for leading the signal light transmitted through the opticalstorage medium 100′ to the photo detector 36, has the same structure asthe liquid crystal device 10 provided on the irradiation optical systemside for applying laser beams to the optical storage medium 100′. Adriving circuit 51, of which a structure is the same as the drivingcircuit 50 for driving the liquid crystal device 10 provided on theirradiation optical system side, drives the liquid crystal device 32 tocorrect the divergent side spherical aberration according to a depthlooking from the condenser lens 31 side, on the condensing point in theoptical storage medium 100′.

In this manner, the optical storage device of the present invention isapplicable also to a transmission detection type of optical storagemedium, and a multi-layer type of optical storage medium as well.

FIG. 20 is a schematic construction view of an optical storage deviceaccording to the sixth embodiment of the present invention.

The optical storage device shown in FIG. 20 is provided with anirradiation optical system for applying a condensing spot to a recordingpoint of the multi-layer type of optical storage medium 100′ also shownin FIG. 19, but no pick-up optical system for reading informationrecorded on the optical storage medium 100′.

In this manner, the optical storage device of the present invention canalso be constructed as a writing-dedicated device.

According to the present embodiments as mentioned above, there have beenexplained all the optical storage device for accessing an optical disk.However, the present invention is not restricted to a disk type ofoptical storage medium, and it is acceptable that an optical storagedevice of the present invention is constructed as an optical storagedevice for accessing another type of optical storage medium, forexample, a tape-like shaped medium. Further, it is sufficient for theoptical storage device of the present invention to access using light,and regardless of principles of information recording and reading, theoptical storage device of the present invention is widely applicable to,for example, a phase change type of optical disk, an optical magneticdisk and the like.

As mentioned above, according to an optical storage medium of thepresent invention, it is possible to effectively correct an aberrationowing to a variation in depth from a surface of an optical storagemedium to a point for condensing, and thereby forming a small opticalspot. Therefore, it is possible to contributing to a higher density ofrecording.

While the present invention has been described with reference to theparticular illustrative embodiments, it is not to be restricted by thoseembodiments but only by the appended claims. It is to be appreciatedthat those skilled in the art can change or modify the embodimentswithout departing from the scope and sprit of the present invention.

What is claimed is:
 1. An optical storage device comprising: a lightsource; an irradiation optical system for leading light emitted fromsaid light source to condense on a predetermined optical storage medium;a photo detector for picking up a signal light carrying informationstored in said optical storage medium to read the information, saidsignal light being condensed onto said optical storage medium andreflected on said optical storage medium; a pick-up optical system forleading said signal light to said photo detector; a liquid crystaldevice having first and second liquid crystal layers disposed midway ofan optical path of said irradiation optical system and extending inparallel with a direction intersecting said optical path, a plurality offirst electrodes for driving said first liquid crystal layer, saidplurality of first electrodes extending a predetermined x-directionintersecting said optical path and arranged in a y-directionintersecting both said optical path and said x-direction, and aplurality of second electrodes for driving said second liquid crystallayer, said plurality of second electrodes extending the y-direction andarranged in the x-direction; a π/2 plate between said first and secondliquid crystal layers; and a liquid crystal driver for applyingcontrolled voltages to said plurality of first electrodes and saidplurality of second electrodes of said liquid crystal device to correctan aberration of light to be condensed on said optical storage medium.2. An optical storage device according to claim 1, wherein saidirradiation optical system has an objective lens at a place adjacent tosaid optical storage medium, said objective lens comprising aplano-convex lens and an aspherical lens.
 3. An optical storage deviceaccording to claim 2, wherein a distance between said objective lens andsaid liquid crystal device is changed in said optical path.
 4. Anoptical storage device according to claim 1, wherein said liquid crystaldriver applies voltages to said plurality of first electrodes and saidplurality of second electrodes of said liquid crystal device, saidvoltages being controlled in such a manner that a phase distribution oflight passing through said first liquid crystal layer in the y-directionis of a Kinoform structure and a phase distribution of light passingthrough said second liquid crystal layer in the x-direction is of aKinoform structure.
 5. An optical storage device according to claim 1,wherein said first and second liquid crystal layers of said liquidcrystal device are determined in their properties in such a manner thata normal of an altering surface of a liquid crystal molecular alignmentin said first liquid crystal layer, due to a change in an electric fieldwithin said first liquid crystal layer according to changes of voltagesapplied to said first electrodes, and a normal of an altering surface ofa liquid crystal molecular alignment in said second liquid crystallayer, due to a change in an electric field within said second liquidcrystal layer according to changes of voltages applied to said secondelectrodes, are directed to a same direction.
 6. An optical storagedevice according to claim 1, wherein said first and second liquidcrystal layers of said liquid crystal device are determined in theirproperties in such a manner that a normal of an altering surface of aliquid crystal molecular alignment in said first liquid crystal layer,due to a change in an electric field within said first liquid crystallayer according to changes of voltages applied to said first electrodes,and a normal of an altering surface of a liquid crystal molecularalignment in said second liquid crystal layer, due to a change in anelectric field within said second liquid crystal layer according tochanges of voltages applied to said second electrodes, establish apredetermined angle, and a wavelength plate for rotating a polarizationdirection of an incident light by the predetermined angle is disposedbetween the first liquid crystal layer and the second liquid crystallayer.
 7. An optical storage device according to claim 1, whereinproperties in alignment of liquid crystal molecules of said first andsecond liquid crystal layers of said liquid crystal device are of bend.8. An optical storage device according to claim 1, wherein saidirradiation optical system is an optical system which permits lightbeams emitted from said light source to pass through said liquid crystaldevice by one time while the light beams are condensed on said opticalstorage medium, and said first and second liquid crystal layers of saidliquid crystal device are set up in their thickness such that phases oflight emitted from said light source and passing through said first andsecond liquid crystal layers vary between 0 and 2π under control ofvoltages applied to said first electrodes and said second electrodes,respectively.
 9. An optical storage device according to claim 1, whereina width of each of said first electrodes of said liquid crystal devicein connection with the y-direction has a size not less than a thicknessof said first liquid crystal layer, and a width of each of said secondelectrodes of said liquid crystal device in connection with thex-direction has a size not less than a thickness of said second liquidcrystal layer.
 10. An optical storage device according to claim 1,wherein a part of said irradiation optical system is shared with a partof said pick-up optical system, said liquid crystal device is disposedat a portion for common use of said irradiation optical system and saidpick-up optical system, light beams emitted from said light source arecondensed via said liquid crystal device onto said optical storagemedium, and the signal light carrying information stored in said opticalstorage medium, which is condensed onto said optical storage medium andreflected on said optical storage medium, is led via said liquid crystaldevice to said photo detector.
 11. An optical storage device accordingto claim 1, wherein: said plurality of first electrodes are the onlyelectrodes driving said first liquid crystal layer, such that said firstliquid crystal layer lacks electrodes extending in the y-direction; andsaid plurality of second electrodes are the only electrodes driving saidsecond liquid crystal layer, such that said second liquid crystal layerlacks electrodes extending in the x-direction.
 12. An optical storagedevice according to claim 1, wherein said electrodes are strip-shapedelectrodes.
 13. An optical storage device according to claim 1, whereinsaid λ/2 plate changes a wave front of a laser beam by rotating apolarization direction of an incident light passing through said firstand second liquid crystal layers.
 14. An optical storage devicecomprising: a light source; an irradiation optical system for leadinglight emitted from said light source to condense on a predeterminedoptical storage medium; a photo detector for picking up a signal lightcarrying information stored in said optical storage medium to read theinformation, said signal light being condensed onto said optical storagemedium and emitted from said optical storage medium; a pick-up opticalsystem for leading said signal light to said photo detector; a liquidcrystal device having first and second liquid crystal layers disposedmidway of an optical path of said irradiation optical system andextending in parallel with a direction intersecting said optical path, aplurality of first, electrodes for driving said first liquid crystallayer, said plurality of first electrodes extending a predeterminedx-direction intersecting said optical path and arranged in a y-directionintersecting both said optical path and said x-direction, and aplurality of second electrodes for driving said second liquid crystallayer, said plurality of second electrodes extending the y-direction andarranged in the x-direction; a λ/2 plate between said first and secondliquid crystal; and a liquid crystal driver for applying controlledvoltages to said plurality of first electrodes and said plurality ofsecond electrodes of said liquid crystal device to correct an aberrationof light to be condensed on said optical storage medium.
 15. An opticalstorage device according to claim 14, wherein: said plurality of firstelectrodes are the only electrodes driving said first liquid crystallayer, such that said first liquid crystal layer lacks electrodesextending in the y-direction; and said plurality of second electrodesare the only electrodes driving said second liquid crystal layer, suchthat said second liquid crystal layer lacks electrodes extending in thex-direction.